Page Tables - aryanjoshi0823/5143-Operating-System GitHub Wiki

In operating systems, page tables are essential data structures that facilitate the mapping of a process's virtual addresses to physical memory addresses. This mechanism enables processes to operate within their own isolated address spaces, thereby enhancing both security and stability.

Key Components of a Page Table:

  1. Page Number: Extracted from the higher-order bits of the virtual address, this number serves as an index into the page table.

  2. Page Table Entry (PTE): Each entry contains crucial information, including:

    • Frame Number: Identifies the specific physical memory frame corresponding to the virtual page.
    • Control Bits: These may include:
      • Present/Absent Bit: Indicates whether the page resides in physical memory or has been swapped out.
      • Protection Bits: Define access permissions, such as read, write, or execute.
      • Dirty Bit: Signals if the page has been modified since it was loaded into memory.
      • Reference Bit: Shows whether the page has been accessed, aiding in page replacement decisions.

  3. Page Offset: Derived from the lower-order bits of the virtual address, this offset specifies the exact byte within the page.

Address Translation Process:

When a process attempts to access a memory location, the system performs the following steps:

  1. Virtual Address Breakdown:

    • The virtual address is divided into two components:
      • Page Number: Directs to the appropriate entry in the page table.
      • Page Offset: Denotes the specific location within the page.

  2. Page Table Lookup:

    • The operating system consults the page table using the page number to find the corresponding PTE.
    • If the Present Bit is set, the page is in physical memory, and the frame number is retrieved.
    • If the Present Bit is not set, a page fault occurs, prompting the operating system to load the page from secondary storage into physical memory.

  3. Physical Address Calculation:

    • The frame number from the PTE is combined with the page offset to form the complete physical address.

  4. Memory Access:

    • The system accesses the physical memory at the calculated address to retrieve or store data.

Illustrative Diagram: